Tuesday, December 18, 2007

6.3 20/20 or bust

So how do we achieve the best corrected vision - without the wavefront, for now? Objective and subjective refraction, naturally.

The techniques described below are collectively known as refraction. A proficient and experienced refractionist usually is the one able to provide the patients with the best Rx in the least amount of time. And the equipment is simply a retinoscope plus a trial lens set or a phoropter - or more conveniently, an auto-refractor for objective refraction.

There are automated visual analysis systems that perform both objective and subjection refraction at the same time. These instruments, however, have not become popular and are not commonly available. Most refraction is still done manually, first objective, then subjective:

1. Objective refraction

Objective refraction does not require the patient’s response. It is especially useful for the cycloplegic refraction of apprehensive babies and uncooperative children. The tools needed are (1) a trial frame with trial lenses or more conveniently, a phoropter; and (2) a streak retinoscope (or a less frequently used spot retinoscope).

The brightness of the retinoscope light source must be adequate as dim lights do not allow accurate assessment of the pupillary light reflex. The batteries (in DC models) and the light bulbs therefore must be checked and replaced regularly. In general, halogen bulbs give more intense lights hence are preferred. The best available retinoscope to date remains the Copeland streak model.

Regardless of the refractive error, the starting lens is always the +1.50D lens. The ideal working distance is 66cm (distance between the refractionist’s eye and the patient’s eye spaced by the fully extended refractionist’s arm to reach the phoropter or the trial lens), power deduction is therefore 1.50D. Shine the retinoscope light into the patient's pupil and move the scope side to side (i.e., by rotating the handle slightly). The refractionist’s right eye is used to refract the patient’s right eye and the left for left. Watch for the change in light reflex: if against motion, add minus power (or reduce plus power) and if with motion, do the reverse. The endpoint is when the reflex motion is neutral which is often when the brightness is the greatest.

In general,(1) Do the primary meridian first (i.e., the least minus direction) using spherical lenses, then use cylindrical lenses for secondary meridian refraction. These two meridians are perpendicular to each other.(2) Go from over-plus, then reduce the power in order to avoid accommodation especially in children – this applies to cycloplegic refraction as well. (3) Occasionally, there are unusual reflexes, e.g., the so-called “scissors” motion, that occur especially in fully dilated pupils. In this case, look for reflexes in the central portion of the pupil and ignore the peripheral reflexes. (4) Deduct 1.50D to reach the preliminary Rx.

The advent of auto-refractors has greatly simplified objective refraction. These instruments are in general quite accurate except in the presence of, e.g.(1) excessive accommodation, especially in children,(2) mydriatic pupils causing spherical aberration,(3) unstable fixation, and(4) opacities in ocular media or silicone oil/gas bubble in the vitreous - for these cases, retinoscopy is far more informative and accurate.

For now, auto-refractor readings, as that from retinoscopy, are used as the starting point for subjective refraction. The skill of retinoscopy itself always will be needed especially in the case of over-refraction, in pediatric cases, and when a reliable auto-refractor is unavailable.

2. Subjective refraction

It is possible to perform subjective refraction without data from retinoscopy or auto-refraction, but the process will be slow and the results not as accurate. This is especially true when refracting high astigmats (for example, >2.5D cylinder) because the spherical equivalents can mask this type of refractive error. Also, the tendency to over-minus is great, because the myopic eye can accept unnecessary minus power yet still with good acuity. Over-minus can stimulate accommodation hence are undesirable as it often leads to headaches and can potentially promote myopia progression in children. A good alternative is to use the patient’s own spectacle power (if it provides good visual acuity) as the starting point and then either over-refract or further refine. In addition, one can assume a 20/40 visual acuity will require a –0.75 to –1.00 D lens and 20/200 will probably need at least –2.00 D of correction. It should be noted that since subjective refraction is based on the patient’s response, it is necessary to ensure reproducibility. The refractionist must often remind the patients not to squint and to answer queries honestly.

Subjective refraction as a rule should not deviate too greatly from the objective finding or one of the two processes is in error.

(1) Refract the right eye first. Some refractionists prefer covering the eye not being examined, others prefer “fogging” (i.e., over-plus by 1.5 to 2D to reduce consensual accommodation). Children do have a tendency to over-accommodate even under cycloplegia. Fogging therefore is needed even under cycloplegia.

(2) For spherical power determination, the least-minus spherical power with the maximal visual acuity is the endpoint. The vision may not be ideal at this point if significant amount of astigmatism is present.

(3) Testing of astigmatism is based on (i) the fan chart followed by (ii) refinement with the Jackson cross-cylinder.

Principles: The patient’s perception of the clearest “spokes” on a fan chart can be used to calculate the cylindrical axis. The clearest lines at certain clock position (from 12 to 6 o’clock) times 3 = axis of astigmatism correction. For example, if the 2 to 8 o’clock line is the clearest, then the axis is 2x3=60 degrees and if 12-6 o’clock, then 6x3=180 degrees.

The Jackson cross-cylinder has both plus and minus cylinders in perpendicular. Flipping-over changes minus to plus cylinder and the reverse from plus to minus but without changing the axis. It is used for cylindrical axis and power refinements. For axis refinement, the principle is that when two cylinders of opposite signs are combined obliquely, the resulting axis is 45 degrees away from the midline of the two cylinder axes. The patient will report clearer view of the visual acuity chart when the minus axis of the cross cylinder is closer to the plus axis of the astigmatism cylinder. The endpoint is when neither looks clear when the cross cylinder is in normal and flipped-over positions. The amount of cylinder power is then adjusted by first superimposing the minus axis of the cross cylinder with that of the astigmatism cylinder and then compared the clarity with that when the cross cylinder is flipped over to the plus side. The endpoint is when both positions look equally clear. Alternatively, the cylindrical lens can be rotated until clear vision, refinement with the cross cylinder will then be much easier.

(4) Duo-chrome test. This is also known as the red-green test. A red-green filter is superimposed onto the acuity chart. Because of chromatic aberration of the eye, the red light is focused in the front and the green light in the back of the retina. Letters on the red side therefore will appear clearer when under-minused and the green side clearer when over-minused. The difference is usually around 0.25-0.50 diopter. The endpoint is when letters on both sides appear equally clear or when the red side is slightly clearer.

(5) Equalization of the two eyes: As the last test, the vision in both eyes must be equalized or the patient will complain of one eye blurrier than the other.

(6) Determination of Adds: Adds (i.e., additions) are plus power added onto the distance Rx in order to compensate for loss of accommodation in the presbyopes. It appears as the segment portion (known as the Seg) of the bifocals or the reading glasses if distance correction is not needed. The Add power can be decided by the Amplitude of Accommodation (with the push-up test) or the Negative Relative Accommodation (measurement of the tolerance to increasing minus power at 40cm). Or more practically, by using loose plus lenses (range: +0.75 to +3.00D) over the distance Rx to achieve a J1+ reading vision at a distance of 40cm or whatever the patient’s preferred reading distance is.

Some patients will need trifocals; the intermediate Add power is usually ½ of the near Add. There are also now the popular progressive bifocals; however, care must be taken to fit the frame properly or the optical center may change its position relative to the eye due, for example, to frame slippage down the nose. Then the patient will have problems seeing clearly. Also the upper border of the Seg is usually set at 1-2mm below the lower lid margin. Too high or too low will both cause discomfort, e.g., severe neck muscle strain.

(7) Additional considerations:

(i) PD (inter-Pupillary Distance): PD is the distance between the two eyes. The optical centers of the two spectacle lenses must match the PD or a prismatic effect will result. Unnecessary prisms are also a cause of eye fatigue and sometimes headaches. Measurement of the PD is usually done with a ruler. More elaborate PD-meters also are available. The near PD is usually 4mm less than the distant PD used in the fabrication of bifocals.

(ii) Phoria and duction measure the resting position of the eyes and the convergence/divergence capabilities, respectively. Usually phoria measurements are used in spectacle prescription in the form of the prism. This is done by first using prism lenses to dissociate fusion and then measure the amount of deviation in the horizontal (exo- and eso-phoria) and vertical (hyper- and hypo-phoria) meridians. The normal range is 2 prism diopters exo and 5 prism diopters eso. The tolerance for vertical phorias is almost none, any amount therefore should be corrected – done with base-up (BU) and base-down (BD) prisms. Base-out (BO) prisms are for the treatment of esophoria and base-in (BI) for exophoria. The prism power is to be divided equally for the two eyes. Notice phorias cannot be corrected with contact lenses which do not incorporate prisms at all.

(8) The final Rx should be written in the minus cylinder form in the following format:

Note: to convert minus- into plus-cylinder form, simply1. Add sphere and cylinder = new spherical power2. Change the sign of cylinder from - to +3. Add/subtract 90 degrees from the axis4. Example: -1.25=-0.75x50 is the same as -2.00=+0.75x140